This invention relates to an apparatus for measuring electron temperature in plasma and particularly to an apparatus for measuring electron temperature with high reliability and in a short time by inserting a probe into plasma.
FIG. 2 shows characteristics of probe current to probe voltage in the case that the distribution of velocity of electrons in plasma obeys Maxwell's distribution. FIG. 3 shows the characteristics using a log-scale for the current. The region where the voltage is lower than a floating potential V.sub.f at which the probe current is zero is the ion saturation current region, the region between the floating potential V.sub.f and the space potential as a turning point in FIG. 3 is the electron retarding region and the region where the voltage is higher than the space potential is the electron saturation current region. In the electron retarding region, the probe current is represented by the following equation: EQU ip=J exp[-e (Vs-Vp)/kTe] (1)
where ip is the probe current, Vp is the probe voltage, Vs is the space potential, e is charge on one electron, k is Boltzmann's constant, Te is electron temperature and J is the electron saturation current determined by the charge and mass of the electron, the shape of a probe electrode and the electron temperature and so forth.
From Eq. (1), with regard to the electron temperature Te, we can obtain the following equation: ##EQU1## where .DELTA.Vp is the difference of the probe voltages, .DELTA.log ip is the difference of logarithmic values of the probe currents while the probe voltage is varied by .DELTA.Vp.
Electron temperature has usually been obtained by a method (1) of measuring the characteristics of the logarithmic probe current to the probe voltage and reading the slope or a method (2) of superimposing an alternating voltage on the probe voltage, detecting change of the probe current and obtaining a ratio of a logarithmic value of the difference to the alternating voltage. However, method (1) needs long measuring times and human judgment and method (2) has a time lag due to the time delay and poor response of the detecting circuit. Method (2) also lacks reliability because the measurement region may go out of the electron retarding region. Moreover, methods (1) and (2) have a common problem in that only low temperature plasma is measurable and the measurable region of the temperature is narrow.
In general, the characteristics of the probe voltage to the current can be divided into the three regions, that is, the ion saturation current region, the electron retarding region and the electron saturation current region. It should be noted that Eq. (1) is satisfied only in the electron retarding region. In the other regions, the electron temperature obtained by Eq. (2) leads to a wrong result.
Conditions of plasma in either laboratry systems or industrial systems widely change depending on kinds of gases, pressure, discharge conditions and so forth. The changes of the plasma conditions influence the probe characteristics and result in a shift of the electron retarding region, change of the electron temperature and change of the density of the electrons.